It seems strange to make pastoral confessions a necessary part of preaching, however, the man's strengths, weakness, and graces should be a visible part of his ministry as a whole.
"But the servant of Christ, whatever his method be, is to perform his whole ministry:
6. With loving affection, that the people may see all coming from his godly zeal, and hearty desire to do them good. And,
7. As taught of God, and persuaded in his own heart, that all that he teacheth is the truth of Christ; and walking before his flock, as an example to them in it; earnestly, both in private and publick, recommending his labours to the blessing of God, and watchfully looking to himself, and the flock whereof the Lord hath made him overseer: So shall the doctrine of truth be preserved uncorrupt, many souls converted and built up, and himself receive manifold comforts of his labours even in this life, and afterward the crown of glory laid up for him in the world to come." The Directory for the Public Worship of God
In other words,, the minister is to be himself, and not present himself as a caricature of what people think a minister should be. He is not a rock star that must always present an image that his fans expect.
As an inexperienced pastor I was often tempted to hide my own struggles for fear that the congregation would lose faith in my abilities. I was tempted to put on airs for the sake of strengthening my ministry.
Obviously, it is better to "take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for when I am weak, then am I strong."